The U.S. Nuclear Regulatory Commission (NRC) said it was stepping up oversight at Constellation Energy Nuclear Group's Nine Mile Point 1 and Entergy (NYSE: ETR)'s Fitzpatrick nuclear power plants in New York as a result of changes in indicators used by the NRC to assess performance at reactors.
The color-coded performance indicators start with “Green” and then increase to “White,” “Yellow” or Red.”
Nine Mile Point Unit 1 had 3.5 scrams or shutdowns during the fourth quarter of 2012, which changed its status from “Green” to “White.” If a plant has more than three unplanned scrams per 7,000 hours of operation, the status changes.
At the Fitzpatrick plant, the indicator was tracking the number of unplanned power changes exceeding six per 7,000 hours of operation. Fitzpatrick’s rolling average was 6.5.
The increased over sight at the plants include supplemental inspection at each site to assure that plant operators are addressing the concerns.
Newest Pilgrim Inspection Report and playing musical chairs during reactor startup.
NRC Violations: Grand Gulf 8, Cooper 11 and Arkansas 9...
Arkansas Nuclear Plant Two
IR 2012005
On August 8, 2012, while at 100 percent reactor power, Unit 2 auxiliary operators secured the 2C-5B condenser vacuum pump in order to perform biweekly oil checks. Control room operators placed the pump in pull-to-lock due to a previously identified issue with the pressure switch that had not been corrected. Unknown to the auxiliary operator, two solenoid valves, 2SV-0690 and 2SV-0688, failed to reposition as designed. The failure of the valves to reposition provided a suction path for the backup vacuum pump through 2C-5B instead of the condenser. Condenser back pressure rose quickly and the control room operators had some difficulty in restarting the 2C-5B pump because it was stuck in the pull-to-lock position. Subsequently, operators were able to restart the vacuum pump, but it was not in sufficient time to prevent a turbine trip on high condenser pressure, resulting in a reactor trip. The event was entered into the corrective action program as Condition Report CR-ANO-2-1012-1429.
The licensee performed a root cause evaluation. The identified root cause was an inadequate design change that called for the installation of the ASCO 8342 model solenoid valves. These solenoid valves were not designed to operate in a high temperature environment such as exists in the turbine building. The licensee further determined between 2008 and 2012 several condition reports were initiated identifying degraded (sluggish) performance of solenoid valves associated with the Unit 2 vacuum pumps. The auxiliary operators were identifying degraded solenoid valve performance within four to six months after installation of the new valves.
The original preventive maintenance plan was to replace the solenoid valves every three years. The condition reports written from 2008 until March 2010 were all categorized as category D condition reports which were closed to work requests. Condition Report CR-ANO-2-2010-0544 was written and categorized as category C following the identification of several previous condition reports that stated similar conditions. The only corrective action stemming from Condition Report CR-ANO-2-2010-0544 was to change the preventive maintenance replacement of the solenoid valves to every eighteen months. This proved to be ineffective as the solenoid valves failed, resulting in the events of August 8, 2012.
LER 2012-001-00
On August 08, 2012 at 0823 CDT, Arkansas Nuclear One Unit-2 (ANO-2) tripped from approximately one hundred percent power due to degraded main condenser vacuum. The 2C-5B Condenser Vacuum Pump (one of two condenser vacuum pumps) had been secured by taking the control handswitch to pull-to-lock by Operations personnel to perform routine oil level checks. Ambient temperatures were low enough to maintain condenser vacuum with one condenser vacuum pump (2C-5A) in service. When 2C-5B was secured, two solenoid valves failed to reposition the isolation valves on 2C-5B, which resulted in a significant air flow path from atmosphere through the vacuum pump, causing condenser pressure to increase to the main turbine trip setpoint. The main turbine tripped on high turbine exhaust pressure which resulted in an automatic reactor scram due to high reactor coolant system pressurizer pressure.
The root cause investigation determined that the subject solenoid valves were installed in an environment with temperatures in excess of designed temperature ratings. This condition resulted in heat related binding of the solenoid valves and failure to reposition when de-energized. The planned corrective action to preclude recurrence of this root cause will implement a modification to change the solenoid valve location to allow proper heat dissipation.
Event Cause
The root cause of the event was determined to be a simultaneous failure of the two ASCO model 8342 solenoid valves [SH][SOL] (ANO-2 component numbers 2SV-0688 and 2SV-0690) which were installed in an environment outside of designed temperature limits, resulting in a heat related failure. The vacuum pump solenoid valves are located inside an enclosed cabinet which is mounted as part of the vendor supplied pump skid. The cabinet doors are normally closed and the enclosure is not ventilated to allow heat generated by the solenoids to escape. The vendor tech manual indicates that the solenoid valves were suited for ambient conditions between 32 and 125 degrees F. The internal cabinet temperature near the valve bodies was measured at approximately 150 degrees F, with solenoid coil temperatures well over 200 degrees F. The internal elastomers were not rated for these temperatures and resulted in valve binding or sluggishness when the solenoids were de-energized.
Corrective Actions
The solenoid valves were replaced prior to plant startup. Future planned corrective action to prevent recurrence of the root cause is expected to include a modification to change the solenoid valve location to allow proper heat dissipation. Additionally, a modification is under consideration that would install a switch or button to verify proper operation of the solenoids and suction valves prior to securing a vacuum pump.
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